Abstract

This paper reports for the first time phreatomagmatic deposits and preserved Surtseyan tuff cones in the Chatham Islands, New Zealand. Fieldwork has located the relicts of at least six, closely-spaced, Paleogene Surtseyan-cones and associated volcaniclastic sediments within the Red Bluff Tuff Formation. The complete stratigraphic section of the cones consists of two parts: 1) the lower part represents volcanic aggradational processes that constructed tuff cones in a short period of time, and is composed of a bedded interval of explosively fragmented, vesicular glassy basaltic pyroclasts (ash and lapilli sizes) as well as feeder-dykes, pillow-lavas and pillow-sills and 2) the upper part represents the rapid denudation of these cones by shallow marine currents or gravity-flows reflecting the instability of the tephra-pile forming the cones, and a much later marine faunal colonization stage (e.g. corals, brachiopods, molluscs, etc.). Erosion could have occurred almost immediately after (or even during) the emplacement of the volcanic pile, similar to what occurred at Surtla vent, a satellite submerged cone of the basaltic island volcano Surtsey, Iceland; the Waiareka-Deborah Volcanics Bridge Point, Aorere Point, and Lookout Bluff Surtseyan-cones (Otago, New Zealand); and Marion and Prince Edward islands (Southwest Indian Ocean), amongst others. By contrast, the complete faunal colonization and stabilization of a diverse marine community could have taken hundreds, or perhaps even thousands of years to reach their acme following the volcanic pulses. The structural, textural and compositional characteristics of the Red Bluff Tuff Formation support a phreatomagmatic mode of fragmentation similar to that at Surtsey Volcano, Iceland. The Red Bluff Tuff sequence represents one of the most complete marine tuff cones described in the geologic record.